FSR2 temporal upscaling fixes: unjittered reprojection, sharpen Y-flip, MSAA guard, descriptor double-buffering

- Motion vectors: single unjittered reprojection matrix (80 bytes) instead of
  two jittered matrices (160 bytes), eliminating numerical instability from
  jitter amplification through large world coordinates
- Sharpen pass: fix Y-flip for correct UV sampling, double-buffer descriptor
  sets to avoid race with in-flight command buffers
- MSAA: auto-disable when FSR2 enabled, grey out AA setting in UI
- Accumulation: variance-based neighborhood clamping in YCoCg space,
  correct history layout transitions
- Frame index: wrap at 256 for stable Halton sequence

assets/shaders/fsr2_motion.comp.glsl

@@ -6,10 +6,8 @@ layout(set = 0, binding = 0) uniform sampler2D depthBuffer;
 layout(set = 0, binding = 1, rg16f) uniform writeonly image2D motionVectors;
 
 layout(push_constant) uniform PushConstants {
-    mat4 invViewProj;       // Inverse of current jittered VP
-    mat4 prevViewProj;      // Previous frame unjittered VP
+    mat4 reprojMatrix;      // prevUnjitteredVP * inverse(currentUnjitteredVP)
     vec4 resolution;        // xy = internal size, zw = 1/internal size
-    vec4 jitterOffset;      // xy = current jitter (NDC), zw = previous jitter
 } pc;
 
 void main() {
@@ -20,25 +18,18 @@ void main() {
     // Sample depth (Vulkan: 0 = near, 1 = far)
     float depth = texelFetch(depthBuffer, pixelCoord, 0).r;
 
-    // Pixel center in NDC [-1, 1]
+    // Pixel center in UV [0,1] and NDC [-1,1]
     vec2 uv = (vec2(pixelCoord) + 0.5) * pc.resolution.zw;
     vec2 ndc = uv * 2.0 - 1.0;
 
-    // Reconstruct world position from depth
+    // Clip-to-clip reprojection: current unjittered clip → previous unjittered clip
     vec4 clipPos = vec4(ndc, depth, 1.0);
-    vec4 worldPos = pc.invViewProj * clipPos;
-    worldPos /= worldPos.w;
-
-    // Project into previous frame's clip space (unjittered)
-    vec4 prevClip = pc.prevViewProj * worldPos;
+    vec4 prevClip = pc.reprojMatrix * clipPos;
     vec2 prevNdc = prevClip.xy / prevClip.w;
     vec2 prevUV = prevNdc * 0.5 + 0.5;
 
-    // Remove jitter from current UV to get unjittered position
-    vec2 unjitteredUV = uv - pc.jitterOffset.xy * 0.5;
-
-    // Motion = previous position - current unjittered position (in UV space)
-    vec2 motion = prevUV - unjitteredUV;
+    // Motion = previous position - current position (both unjittered, in UV space)
+    vec2 motion = prevUV - uv;
 
     imageStore(motionVectors, pixelCoord, vec4(motion, 0.0, 0.0));
 }